A 4.2Ghz dual will still be more than enough for MOST games but there are a few that are borderline requiring a quad to function, though one of those was GTA4 and I seem to of read that the latest patch actually made the game run far better on dual cores.
I wouldn't think there will be a massive surge forward in cpu requirements for the vast majority of games between now and "next gen" chips from both AMD and Intel.
At the absolute most, I would not upgrade to an expensive quad core system, with what you have I'd say pick up a second hand S775 quad, if you can a Q6600, or really anything, its the 4 cores rather than what cores they are that matters really as you'll still be mostly gpu limited.
I'd say a AMD quad core system would be the next option as a better value way to get a quad system than going to i7, though I'd still go s775 quad if I were you, without question.
Frankly any half decent quad, Q6600/P2 and up will last you, for gaming, till WELL beyond next year, and well into decent value next gen CPU's being released. i7 offers quite literally nothing in terms of gaming as an upgrade, though is great for other things but gaming, quite literally nothing so I'd completely skip it and wait for AMD/Intel offerings from the next gen and the next socket for a big upgrade. AMD's next gen looks really something very very special, Intels seems to be a more minor upgrade but we'll have to see when that gets closer.
But realistically for gaming as I said a Q6600 and up, or really any current AMD Phenom 2 and up will be plenty for gaming and will easily max out any game for the next couple years realistically and maybe beyond.
The current P2 chips hit 4Ghz quite easily with low power usage, low idle power and great temps, i7's quite a power hog, s775 quads are pretty decent for power and overclocking aswell. Theres also the option of dual core p2's unlocked to quad core, and in not too long cheap quad cores unlocking to hex cores on the AMD side if you wanted a budget upgrade to last a couple years.
As for which s775 quad core, in gaming you'd not be able to see a difference between a Q6600 and a Q9xxx chip, I mean, you really couldn't tell which was in there, the Q6600 is one of the best value in terms of how long the chips offered max gaming performance(with overclocking which it does well) and how much longer it will be fine for high end gaming.
